The present invention relates to a method for preparing an NC machining program and an apparatus for preparing an NC machining program.
Conventionally, there has been known an NC machining apparatus for executing a cutting process of a workpiece made of a difficult-to-machine material such as a heat-resistant alloy or the like. As the NC machining apparatus, for example, a description will be given of a case that a workpiece made of the heat resistant alloy is processed by using an NC lathe. At a time of describing the process of the workpiece, X-axis, Z-axis and Z-axis directions are defined respectively as shown in FIGS. 6 to 8.
As shown in FIG. 6, in the case of cutting a workpiece W by using a normal bite 50, since thermal conductivity of the bite 50 is low, heat is not transmitted to the workpiece W from the bite 50, and the heat tends to be accumulated in the bite 50. Accordingly, the structure is made such as to make a rotating speed of the workpiece W lower so as to cut the workpiece W (hereinafter, refer to as a prior art 1).
In accordance with the prior art 1, a cut portion of the bite 50 tends to be exposed to a boundary damage as the bite 50 generates heat. Accordingly, a machining efficiency is low, and a service life of the bite 50 is short. Further, since a part of a rotating force is applied to the bite 50 at a time when the workpiece W is rotated, the bite 50 is bent and the cutting process is not stable.
Further, as shown in FIG. 7, there is a method of cutting the workpiece W by using an end mill 60 (hereinafter, refer to as a prior art 2). In accordance with the prior art 2, it is possible to suppress the heat generated at a time of cutting. However, a profile irregularity of the processed surface of the workpiece W is low, and a machining efficiency is low.
Further, in Japanese Laid-Open Patent Publication No. 10-118803, there is proposed a method of cutting a workpiece W by using a rotary bite. In this case, since a tip of the rotary bite is exposed to a cutting load, the position of the rotary bite is continuously changed. Accordingly, a rotating speed of the rotary bite is determined in correspondence to the cutting load (hereinafter, refer to as a prior art 3). In accordance with the prior art 3, a generation of the boundary damage is suppressed. However, since the rotating speed of the rotary bite is determined in correspondence to the cutting load, it is impossible to obtain a high machining efficiency.
In recent years, as shown in FIGS. 8A and 8B, there has been proposed a technique of fixing a circular tip 140 to a distal end of a rotary tool 120, and cutting a workpiece W by the circular tip 140. In this case, a tool holding portion 150 is provided in a bite holder (not shown), and the rotary tool 120 is attached to the tool holding portion 150. The bite holder can turn around a B-axis which is in parallel to the Y-axis, and can reciprocate in the X-axis, Y-axis and Z-axis directions. The rotary tool 120 is constituted by a tool main body 130 and a circular tip 140. The tool main body 130 is attached to the tool holding portion 150, and the tip 140 is fastened at a center in a distal end of the tool main body 130 by a bolt 132.
(Lead Clearance α)
As shown in FIG. 8A, the circular tip 140 is inclined in the direction of the Z-axis from a standard position shown by straight line L1, specifically, a feeding direction of the circular tip 140. The lead clearance α can be expressed as an angle formed by the straight line L1 and an axis of the circular tip 140.
A description will be given of a reason for setting the lead clearance α with reference to FIG. 10. In FIG. 10, reference symbol C denotes a rotating direction of the workpiece W. In the case that the lead clearance α is 0 degree, the rotary tool 120 tends to be bent at a time of cutting the workpiece W. Accordingly, the cutting of the workpiece becomes unstable, and a profile irregularity of the processed surface is lowered.
Accordingly, when the circular tip 140 is moved in the direction of the Z-axis at a time of cutting the workpiece W, the circular tip 140 is inclined at the lead clearance α (refer to FIG. 8). In this case, an auxiliary component force Fb is applied to the circular tip 140 in the feeding direction of the circular tip 140, and a main component force Fb is applied thereto in a direction orthogonal to the feeding direction. Further, the rotary tool 120 is hard to be bent by applying a combined force F of the respective component forces Fa and Fb in the same direction as the axis of the circular tip 140. Accordingly, a vibration of the rotary tool 120 is suppressed, a service life of the tool becomes longer, and the profile irregularity of the processed surface is improved.
Further, in the case of cutting the workpiece W having a large diameter by setting the lead clearance α to 0 degree, it is necessary to enlarge a protruding amount of the tool from the bite holder so as to avoid interference between the tool and the bite holder. However, in this case, the rigidity of the tool is lowered. Accordingly, if the lead clearance α is set to an angle other than 0 degrees, it is possible to make the protruding amount of the tool small, and it is possible to make the rigidity of the tool higher.
(Radial Clearance β)
As shown in FIG. 8B, the circular tip 140 is arranged in an upper side of center line O of the workpiece W. The radial clearance β is expressed as an angle (a cutting angle) formed by a cutting surface M including center line O of the workpiece W and a cutting portion 140a of the circular tip 140 and the Y-axis. Stated another way, and also as shown in FIG. 8B, the cutting surface M can represent a line M from the center line O of the workpiece W to a point of contact of the rotating tool 120 against the workpiece W during the cutting operation. The radial clearance β is the angle between the line M and the Y-axis.
A description will be given of a reason for setting the radial clearance β with reference to FIG. 9. In the case that the radial clearance β is 0 degrees, a flank 142 of the circular tip 140 is undesirably brought into contact with the workpiece W so as to wear.
Accordingly, a clearance is secured between the flank 142 of the circular tip 140 and the workpiece W by setting the radial clearance β at a time of cutting the workpiece W. Therefore, an abrasion loss of the flank 142 of the circular tip 140 is reduced, and a service life of the tool is elongated.
Further, the abrasion loss of the flank is reduced even in a negative tip, in which a relief angle of a cutting edge is 0 degrees, by changing the radial clearance β so as to adjust an amount of the clearance between the circular tip 140 and the workpiece W. Further, in the case that a movable distance in the direction of the Y-axis of the circular tip 140 is small, at a time of cutting a workpiece W having a large diameter, it is preferable to enlarge the radial clearance β.
As mentioned above, the abrasion loss of the circular tip 140 is reduced by adjusting the lead clearance α and the radial clearance β so as to cut a workpiece W, and the profile irregularity of the processed surface is improved.
In accordance with the prior arts 1 to 3, the NC machining program is prepared in such a manner that the rotary tool moves in the direction of the X-axis and the direction of the Z-axis. In this case, since a coordinate position of the rotary tool is expressed only by an X-Z two-dimensional coordinate, it is possible to easily prepare the NC machining program.
However, as shown in FIGS. 8A and 8B, in the case that a workpiece W is cut by using the circular tip 140, it is necessary that the coordinate position of the tool is expressed by an X-Y-Z three-dimensional coordinate. In this case, a structure of the program becomes complicated, and it is impossible to easily prepare the NC machining program.